Did you arrange a final read of the meter(s) ?
If so, then of course you should not receive anything past your last bill.
If not, did you take a photo of your meter(s) reading(s) ?
If so, you might be able to send this with timestamp showing the last read, but the retailer might not be obligated to use this and keep charging until notified of your leaving the location, and finalising the account.
You might be able to show the new company started charging a new tenant on a particular day / reading, but again it's not up to a retailer to chase this.
You should immediately contact your retailer to find out if your notice to discontinue service was received / acted on, or if you failed to make that notice to them, get a reading done asap / take some photos asap.
Perhaps if you have spoken to the new tenant and they are willing to help you, you could ask them for a copy of their first bill and readings ?
At least a photo of the header with new retailer, and the meter read info.
Call AGL help or use chat asap to advise them the situation, will be faster than reply / messages here.